1. Technical Field
The present invention relates to an optical device, a detection device, and so on.
2. Related Art
In recent years, demand for sensors used, for example, for medical diagnostics or inspections of food and drink has increased, and further, development of a highly sensitive and small sized sensor has been demanded. In order to meet such a demand, a variety of types of sensors such as a sensor using an electrochemical process have been studied. Among these sensors, sensors using surface plasmon resonance (SPR) have been receiving increasing attention on the ground of possibility of integration, low cost, and applicability in all measurement environments.
For example, in JP-A-2000-356587 (Patent document 1), there is disclosed a method of using localized surface plasmon resonance (LSPR) to thereby improve sensor sensitivity.
In “Experimental study of the interaction between localized and propagating surface plasmons” (OPTICS LETTERS/Vol. 34, No. 3/Feb. 1, 2009; Non-patent document 1), there is disclosed a method of using both of propagating surface plasmon (PSP) and localized surface plasmon (LSP) to thereby improve the sensor sensitivity.
As shown in FIG. 1, in the patent document 1, fine metal particles 20 are fixed on a surface of a transparent substrate 10, and the transparent substrate 10 is irradiated with incident light to thereby measure the absorbance of the fine metal particles 20. As shown in FIG. 2, if a target object adheres the fine metal particles 20, a change from an absorbance spectrum indicated by Al to an absorbance spectrum indicated by A2 occurs. According to the method of the patent document 1, a change in a medium in the vicinity of the fine metal particles is detected due to the change in the absorbance to thereby detect adsorption or deposition of the target object.
However, in this method, it is difficult to manufacture the fine metal particles so as to have uniform dimensions and shape, and to regularly arrange the fine metal particles. If the sizes and the arrangement of the fine metal particles fail to be controlled, variations are also caused in absorption and resonant wavelength generated by the plasmon resonance. Therefore, as shown in FIG. 2, the width of the absorbance spectrum becomes broader, and a peak intensity is lowered. Further, if the peak intensity is lowered, a signal variation for detecting the variation in the medium in the vicinity of the fine metal particles becomes smaller, and there arises a limitation in the improvement in the sensor sensitivity. Therefore, in such a usage of identifying a material based on the absorbance spectrum, it results that the sensor sensitivity is insufficient.
Moreover, since a surface enhanced Raman scattering (SERS) sensor in the related art only uses one of resonance peaks, it is required to fit the wavelength of the resonance peak to either one of an excitation wavelength and a Raman scattering wavelength. In this case, it results that only an electric field enhancement effect in either one of scattering processes is used, and therefore, a high electric field enhancement effect cannot be expected.
Meanwhile, in the Non-patent document 2, as shown in FIG. 3, there is disclosed a sensor provided with an Au film 40 having a thickness of 100 nm bonded to a glass substrate 30, an SiO2 layer 50 having a thickness of 20 nm formed on the Au film 40, and a plurality of Au disks 60 each having diameter in a range of 100 through 170 nm arranged two-dimensionally on the SiO2 layer 50 at a pitch P=780 nm.
In the sensor, there is excited the propagating surface plasmon PSP in an interface between the Au film 40 and the SiO2 layer 50, and there is excited the localized surface plasmon LSP in the Au disks 60. Here, the propagating surface plasmon PSP is coupled to an evanescent field having a “wave number.” The “wave number” is determined by the pitch P of the Au disks 60 to 2π/P. Therefore, the pitch P of the Au disks 60 has a correlation with the excitation of the propagating surface plasmon PSP, and therefore, has a correlation with the resonance peak wavelength set in accordance with the Raman scattering wavelength of a sample, and cannot be changed freely.
On the other hand, the Au disks 60 each have a function as a hot site where a localized electrical field is enhanced, and in order for improving the sensitivity of the sensor, it is required for the hot sites to have a high density. However, the pitch P of the Au disks 60 determining the “wave number” of the propagating surface plasmon PSP is as relatively large as 780 nm, and the density of the hot sites becomes remarkably low. However, if an outer dimension of the Au disks 60 is increased while keeping the pitch P large, the resonant wavelength is shifted from the excitation wavelength on the longer wavelength side (the red side), and therefore, a strong localized electrical field cannot be expected.